Elasticity of demand is calculated as %change in quantity demanded / %change in price
If %change in quantity demanded > %change in price, demand is elastic. There is elastic demand of airline industry because it is consumed as a luxury or non-necessary good because people often use it for trips, vacations, travel which can be reduced in hard times when income level falls. Due to fall in Income, consumers demand less of air travel which reduces total revenue of travel industry.
You can see in the graph below that area of portion P1AQ1O (total expenditure when deman reduced) is less than area of portion P2BQ2O (total expenditure before decrese in income).
frut Supply Bi B Demand dz O quantity